Method and structure for graded gate oxides on vertical and non-planar surfaces
A method for forming an oxide layer on a vertical, non-planar semiconductor surface provides a low stress oxide layer having a pristine interface characterized by a roughness of less than 3 angstroms. The oxide layer includes a portion that is substantially amorphous and notably dense. The oxide layer is a graded growth oxide layer including a composite of a first oxide portion formed at a relatively low temperature below the viscoelastic temperature of the oxide film and a second oxide portion formed at a relatively high temperature above the viscoelastic temperature of the oxide film. The process for forming the oxide layer includes thermally oxidizing at a first temperature below the viscoelastic temperature of the film, and slowly ramping up the temperature to a second temperature above the viscoelastic temperature of the film and heating at the second temperature. After the second, high temperature oxidation above the viscoelastic temperature, the structure is then slowly cooled under gradual, modulated cooling conditions.
1. A method for forming an oxide layer on a vertical semiconductor surface comprising:
providing a substrate having a horizontal substrate surface and a substantially vertical semiconductor surface formed over said horizontal substrate surface;
thermally oxidizing said vertical semiconductor surface thereby forming a first oxide film on said vertical semiconductor surface, said thermally oxidizing occurring at a first temperature below the viscoelastic temperature of said oxide film; and
further thermally oxidizing said vertical semiconductor surface thereby forming a second oxide film on said vertical semiconductor surface, said further thermal oxidizing occurring at a second temperature above said viscoelastic temperature,
said first oxide film and said second oxide film forming an oxide layer on said vertical semiconductor surface.
2. The method as in claim 1 , in which said first temperature is within the range of 750–800° C.
3. The method as in claim 1 , further comprising cooling after further thermally oxidizing, said cooling including a relatively slow cooling rate at temperatures above said viscoelastic temperature and a relatively fast cooling rate at temperatures below said viscoelastic temperature, said relatively slow cooling rate being no greater than 10° C./minute.
4. The method as in claim 1 , in which said viscoelastic temperature is about 925° C.
5. The method as in claim 1 , further comprising heating from said first temperature to said second temperature at a rate no greater than 15° C. per minute.
6. The method as in claim 1 , wherein said vertical semiconductor surface comprises silicon and said first and second oxide films are silicon dioxide.
7. The method as in claim 1 , in which said vertical semiconductor surface includes at least one of a convex corner and a curved section.
8. The method as in claim 1 , in which said providing includes forming a plug of one silicon, silicon/germanium, and silicon/germanium/carbon, said plug including opposed ends and said vertical semiconductor surface being a surface of said plug.
9. The method as in claim 8 , wherein said vertical semiconductor surface comprises a transistor channel and said oxide layer comprises a gate oxide.
10. The method as in claim 9 , further comprising forming one of a source region and a drain region at one end of said plug, and the other of said source region and said drain region at the other end of said plug, and forming a polysilicon layer laterally contacting said gate oxide and forming a transistor gate.
11. The method as in claim 1 , wherein said providing comprises providing a vertical silicon plug disposed over said horizontal surface, said vertical silicon plug including at least one generally flat vertical surface and at least one curved vertical surface forming a junction therewith.
12. The method as in claim 1 , further comprising upwardly ramping to said first temperature at a rate greater than 35° C./minute to reduce any oxide formed during said upwardly ramping.
13. The method as in claim 1 , wherein said substrate comprises a semiconductor and said providing includes:
forming a first device region selected from the group consisting of a source region and a drain region of a semiconductor device, in said horizontal substrate surface;
forming at least three layers of material over said first device region, wherein said second layer is interposed between said first and said third layers;
forming a window in said at least three layers of material, wherein said window terminates at said first device region in said horizontal substrate surface;
filling said window with a semiconductor material thereby forming a semiconductor plug, wherein said plug has a first end and a second end and wherein said first end is in contact with said first device region and said plug includes said vertical semiconductor surface;
forming a second device region selected from the group consisting of a source region and a drain region in said second end of said semiconductor plug wherein one of said first and second device regions is a source region and the other is a drain region;
removing a portion of said third layer, thereby exposing said second layer underlying the removed portion of said third layer; and
removing said second layer thereby exposing said vertical semiconductor surface.
14. The method as in claim 1 , in which said vertical semiconductor surface comprises a vertical silicon surface and said providing includes:
forming a plurality of films over said horizontal substrate surface, said plurality of films including a sacrificial layer and an upper layer over said sacrificial layer;
forming an opening through each of said layers, thereby exposing said horizontal substrate surface;
filling said opening with silicon;
removing portions of said upper layer; then
removing said sacrificial layer thereby exposing said vertical silicon surface.
15. A method for forming an oxide layer on a non-planar surface comprising:
providing a non-planar silicon surface;
thermally oxidizing said non-planar silicon surface thereby forming a first oxide film thereon, said thermally oxidizing occurring at a first temperature below the viscoelastic temperature of said oxide film; and
further thermally oxidizing said non-planar silicon surface thereby forming a second oxide film on said non-planar silicon surface, said further thermal oxidizing occurring at a second temperature above said viscoelastic temperature,
said first oxide film and said second oxide film forming a non-planar oxide layer on said non-planar silicon surface.
16. The method as in claim 15 , in which said non-planar silicon surface includes at least one of a convex corner and a curved section.
17. The method as in claim 15 , in which said first temperature is within the range of 750–800° C. and further comprising upwardly ramping to said first temperature at a rate greater than 35° C./minute to reduce any oxide formed during said upwardly ramping.
18. The method as in claim 15 , further comprising cooling after further thermally oxidizing, said cooling including a relatively slow cooling rate at temperatures above said viscoelastic temperature and a relatively fast cooling rate at temperatures below said viscoelastic temperature, said relatively slow cooling rate being no greater than 10° C./minute.
19. The method as in claim 15 , in which said viscoelastic temperature is about 925° C., and further comprising heating from said first temperature to said second temperature at a rate no greater than 15° C. per minute.